Rabbi Aryeh Kaplan in the “Living Torah” translates the end part of Numbers Chapter 25:
16 God spoke to Moses, saying, 17 “Attack the Midianites and kill them
18 since they attacked you through their plot with Peor* as well as through
their sister, Kazbi, daughter of a Midianite prince, who was killed on the
day of plague that resulted from Peor.”
Rabbeinu Bachayei stresses the need to attack the Midianites based on their present state of mind.
Rabbeinu Bachayei Numbers 25:17
Attack the Midianites. After He (Hashem) had given his good reward to the righteous, he commanded to punish the wicked and said to him, “Attack the Midianites”. But not the Moabites on account of Ruth the Moabite that David in the future would descend from her.
And in the midrash (Tanchuma 3) “Attack the Midianites, because they are attacking you“, from here from here our sages of blessed memory deduced, “he that arises to kill you, get up early to kill him”.